This invention relates in general to polyimide foams and more specifically, to making rigid, flame resistant foamed structures from simple precursors with minimal processing.
Polyimides have been important commercial polymers for approximately 20 years, but until recently, have not been readily fabricated as foams. Gagliani et al, in U.S. Pat. Nos. 4,305,796, 4,407,980, and 4,425,441, among others, describe the preparation of resilient foams and structures formed from them, as well as methods of preparing and curing them. Many of the procedures require binary or ternary mixtures of amines, use of microwave or pulsed microwave heating, with or without preheating, postcures, and a 7 step procedure for preparing a foam from a dianhydride, an oxoimine, an alcohol, a surfactant, and an aromatic diamine, with drying, grinding, vacuum drying, and expanding to form a flexible foam (U.S. Pat. No. 4,425,441).
Suitable foams are only produced from relatively large particles (0.5 to 10 mm), preferably 1 to 5 mm diameter. There is a need for a material to be used for forming thin structures, such as resilient tiles or rigid glass cloth covered wall panels less than 5 mm in thickness. Large particles will not even fit in such a mold, let alone produce a uniform, closed cell structure.
For production of rigid molded structures (2 to 5 lbs/cubic foot), no comparable procedure is known. Also, there is a need for reduction in process complexity to allow for more economical production.